1. Field of the Invention:
The present invention relates to a magnetic field pole assembly for generating a magnetic field in a DC motor.
2. Description of the Prior Art:
There has heretofore been known a magnetic field pole assembly of the type described which, as shown in FIG. 1 of the accompanying drawings, comprises a tubular yoke 1 of ferromagnetic material and a plurality of radial permanent magnets 2 mounted in angularly spaced relation in the tubular yoke 1 and each having a pole piece on a distal end thereof, with a cylindrical space 4 defined jointly by the permanent magnets 2 centrally in the tubular yoke 1 for receiving therein a rotor (not shown), there being magnetic paths M.sub.1 formed by the yoke 1 and the permanent magnets 2. The permanent magnets 2 are normally composed of an alnico alloy (4 alnico 5-7). As is well known, the alnico alloy consists chiefly of cobalt, iron and other materials, and is so expensive that only the permanent magnets take about 70% of the cost of the magnetic field pole assembly. The alnico magnets cannot be shaped to contour with high precision.
In order to gain proper magnetic flux distribution in the cylindrical space 4, it is necessary that adjacent permanent magnets be angularly spaced equal intervals around the central axis of the tubular yoke 1. The permanent magnets 2 should be affixed to the pole pieces 3 and the tubular yoke 1 in intimate contact therewith for the formation of the magnetic paths M.sub.1. These requirements result in a high precision required of the shape and dimensions of the permanent magnets 2. There is also required a high degree of concentricity between the magnetic circuit and the rotor (not illustrated) inserted in the cylindrical space 4. To obtain a desired dimensional accuracy, therefore, the pole pieces 3 should be finished on their inner surfaces 5 after the magnetic field pole assembly has been put together.
Another magnetic field pole assembly has been known which is designed to meet the foregoing requirements, particularly the economical requirement. The magnetic field pole assembly includes, as shown in FIG. 2, a tubular frame 6, a plurality of pole pieces 3 constructed of laminated sheets of silicon steel and mounted in angularly spaced relation in the frame 6 with a cetral cylindrical space 4 defined by the pole pieces 3, and a plurality of permanent magnets 2 each in the form of a rectangular parallelopiped located between adjacent ones of the pole pieces 3. The permanent magnets 2 have like polarities confronting each other to form alternate polarities on the inner surfaces 5 of the pole pieces 3 and magnetic paths M.sub.2. With such a magnetic field pole assembly, the permanent magnets 2 can have wider magnetic pole surfaces (effective cross-sectional areas) and hence can increase the efficiency of the magnetic pole assembly, an arrangement which allows the use of less costly ceramic magnets as well as more expensive alnico magnets.
A proper magnetic flux distribution (proper magnetic field balancing) in the cylindrical space 4 requires that adjacent pole pieces 3 be arranged at equal angular intervals around the central axis of the tubular frame 6, and the pole pieces 3 and the permanent magnets 2 form a complete cylindrical body. The magnetic field balancing in the cylindrical space 4 would be impaired if there were gaps between the pole pieces 3 and the permanent magnets 2, and also if the cylindrical body had an increased out-of-roundness. Therefore, the components of the magnetic field pole assembly need to have a high dimensional accuracy and to be precision assembled. As with the magnetic field pole assembly as shown in FIG. 1, there should be a high concentricity between the magnetic circuit and the rotor (not shown) placed in the cylindrical space 4, and the inner pole piece surfaces 5 are required to be finished after the magnetic field pole assembly has been assembled for a sufficient degree of dimensional accuracy.
While the magnetic field pole assembly illustrated in FIG. 2 can incorporate ceramic magnets, they fail to produce a compatible DC motor magnetic field as compared with the magnetic field pole assembly as shown in FIG. 1, which has alnico magnets assembled therein and is of the same dimensions (outside diameter and height) as those of the assembly of FIG. 2.